In the previous session the heroes had cleared most of the ground floor of Castle Ravenloft. This evening we only played like half a session, because we started late, and then took some time to review all the handouts (like the Book of Strahd) and other clues the players had collected up to now. By doing so it became obvious that their next step should be to search for the grave of Count Strahd von Zarovich in the dungeons of the castle and scatter the earth from it to weaken the vampire lord before attacking him.

One helpful tool was that because I had made the maps of the castle myself with Campaign Cartographer, I was able to print out a map of the ground floor of the castle with a "fog of war" effect blocking the corner they hadn't explored yet. They knew from the butler Igor that the dining hall was in that corner, that the vampires were at dinner, and they could hear organ music playing from there. The butler had also gone that way and said that the stairs down to his kitchen were in that dining hall. And as the only other stairway they had found was leading up and not down, the players decided that the dining hall was the place to go.

So the rest of the session was the fight of the party against the vampires in the dining hall. It turned out that I had done a good job of scaring my players appropriately for a gothic horror adventure, and they were positively reluctant to rush into a room full of vampires. So when they opened the door of the dining hall and saw all the vampires at the tables with servants between them and Count Strahd at the organ playing music, they used their round of surprise to close the door again. Now the castle vampires usually have crossbows, but had not taken them to table. The players heard movement from the room, but one full round of combat absolutely nothing happened, with both sides waiting to pounce on whoever would open the door first.

Finally the door opened, but the vampires had sent one of their servants to open it, while the vampires kept out of sight behind the corner. Considering that as a hostile act, the players killed the servant, but didn't close the door again, placing two melee specialists in the doorway, the fighter and the rogue. Judging the vampires to be intelligent, I played them tactically well, and had the first vampire make a bull rush attack on the rogue which succeeded in pushing him back. That allowed the other vampires to trickle into the hallway over the next rounds where a close quarters combat evolved.

One highlight of the fight was that the cleric tried his new daily spell, Spiritual Weapon, which gave every player combat advantage on the enemy the spiritual weapon was hitting. The warlord had an at-will power that increased the combat advantage bonus on whoever he was hitting with it. And the rogue had an improved sneak attack doing 2d8 extra damage whenever he had combat advantage. That proved to be a devastating combo, with the rogue basically hitting with every attack and doing serious damage. The wizard was less well-placed to act in these cramped quarters with his area spells and mostly resorted to magic missiles. The cleric's turn undead worked well in that situation, being a close range area effect. And the fighter with his artifact axe doing radiant damage was also a serious threat, in spite of being hampered by a negative effect of the despair deck preventing him from doing opportunity attacks.

In the end all the vampires lay dead in the hallway, and Strahd was still playing the organ in the dining hall. The players closed the door again, still afraid of the Count, and we ended the session there.

Yes, undead can be hit with sneak attacks and take critical hits like any other mob. In what edition were they generally immune to crits and sneaking? I don't remember that from 1st or 2nd edition, but I didn't play much of 3 or 3.5. And as far as I remember the early editions of (A)D&D didn't have critical hit rules, and everybody had house rules for that.

In 3-3.5 there was a quite large population of crit/sneak attack immune monsters, basically everything that is non-living or doesn't have "a discernible anatomy". Undead, constructs, elementals, incorporeal creatures, oozes, animated objects...